DSP Applications and Student Projects - Digital Signal Processing and Applications with the C6713 and C6416 DSK - page 388

// transmitter.c Transmits voice as a BPSK signal
#include "dsk6713_aic23.h" //codec-DSK support file

#include <math.h>
#include "lp1500.cof" //1500 Hz coeff lowpass filter
Uint32 fs=DSK6713_AIC23_FREQ_48KHZ; //set sampling rate
#define NUMSAMP 4 //# samples per Symbol
#define MAX_DATA_LENGTH 10 //size of Mod/Demod vector
#define NUM_BITS 8 //number of bits per sample
#define SYNC_INTERVAL 100 //interval between sync bits
short encSeqNum = 8; //number of encoded bits
short encSymbolVal = 0; //encoder symbol index
short sin_table[NUMSAMP]={0,1000,0,-1000}; //for carrier
short bits[8]; //holds encoded sample
short sampleBuffer[2000]; //to view sample
short sIndex = 0; //index sampleBuffer
short syncSequence[8]={1,1,1,-1,1,-1,-1,1};//synchronization sequence
short outval=1; //bit value to be encoded

short encodeVal = 0; //filtered input value

int yn = 0; //init filter's output

short gain=10; //gain on output

short syncTimer = 0; //tracks time between syncs

#define LEFT 0 //setup left/right channel

#define RIGHT 1
union {Uint32 uint; short channel[2];} AIC23_data;

interrupt void c_int11() //interrupt service routine
{
int i;
short sample_data;
sample_data = input_sample();
yn = fircircfunc(sample_data,h,N); //asm func passing to A4,B4,A6
if(encSymbolVal >= NUMSAMP)

//increment through waveform

{
encSymbolVal = 0;
encSeqNum++;
}
if(encSeqNum == NUM_BITS) //when all 8 bits sent

{ //get a new sample

encSeqNum = 0;
if(syncTimer++ >= SYNC_INTERVAL) //determine whether
{ //to send sync sequence

syncTimer = 0;
for(i=0; i<8; i++) //put sync sequence in bit

bits[i] = syncSequence[i];
}
else
{ //get the bits

encodeVal = (short) (yn >> 15);
for(i=8; i<16; i++) //encode input sequence
bits[i-8]=(encodeVal&(1<<i)) ? 1 : -1; //shift
}

FIGURE 10.31. C program for BPSK transmission ( transmitter.c ).